The ideal set-up is an oversized/large trans cooler before the "in-tank" cooler that maintains the "running-temp" of the returning fluid to the trans.

Heat is the greatest nemesis of an automatic trans.

And as mentioned, too cool is not ideal either, the fluid is not a consistent temp for operation.

Some car makers now use a thermostatic bypass to help the trans heat up its fluid quicker to running temp......something to think about.